An ordinary programmable controller uses non-volatile memory means such as a ROM storing a specific ladder program in order to make possible sequence control conforming to a controlled system. If the controlled system of the programmable controller is a machine tool, it is required to create a ladder program in such a manner that it will not be necessary to change the existing ROM whenever there is a modification in a specifications of the machine tool. One method of achieving this is to make the portion of the ladder capable of being modified as a subroutine, and storing the subroutine in a rewritable EROM (erasable read-only memory). In modifying a ladder or adding to a ladder, a plurality of ROM boards are prepared in advance and these are plugged into, say, a mother board of the programmable controller, thereby making it possible to modify or add to the contents of a ladder.
In comparison with a case in which the ladder program is stored in temporary storage-type memory means such as a RAM and backing up the RAM using a separate power supply, the above method of storing the ladder program using only a conventional ROM is advantageous in that erasure of the program due to a power supply abnormality or the like can be prevented in reliable fashion. Conversely, however, the speed at which the program data can be read out of the ROM is lower than with the RAM, and therefore the operating speed of the machine tool is restricted by the ladder processing speed.
Though a programmable controller has been considered in which joint use is made of an ultraviolet erasable-type EPROM (erasable and programmable ROM), all of the data stored in single units in such case is erased simultaneously when the data is rewritten, and therefore it is impossible to modify a portion of the ladder. Moreover, another disadvantage is that a ROM unit must be extracted from the mother board on account of this erasure. With conventional programmable controllers of this kind, many problems are encountered in terms of modifying the ladder program and operating an NC (numerical control unit) efficiently.